Why Those Old High-Speed Motors Might Be Costing You More Than You Think

Let's Cut to the Chase: Why This Matters

Look, if you're reading this, you've probably got a plant floor full of old squirrel cage induction motors humming away, and someone just handed you a PO for a Variable Frequency Drive (VFD). And you’re wondering: is pairing these two a no-brainer, or a money pit waiting to happen? In my role coordinating motion control retrofits for manufacturing and logistics clients, I've seen this exact scenario dozens of times. Usually, the answer is more nuanced than a simple 'yes' or 'no'. So, let’s tackle the most common questions I get.

Q: Wait, 'What VFD Stands For' is one of my search terms. Can you just start with the basics?

Honestly, sure. VFD stands for Variable Frequency Drive. It’s an electronic device that controls the speed of an AC motor, like a squirrel cage induction motor, by varying the frequency and voltage of the power supplied to it. It’s the brains that let you slow down or speed up your conveyor, fan, or pump without mechanical gear changes. Think of it as the dimmer switch for your industrial machinery.

Q: I've got a bunch of old Boston Gear speed reducers and motors. Can I just slap a new VFD on any old squirrel cage induction motor?

This is the big one. And the answer is… it depends. I learned this one the hard way. We had a client eager to save money by keeping a 20-year-old motor and just adding a drive. I assumed the motor insulation and cooling fan could handle it. Didn't verify the nameplate data thoroughly enough. Turned out the old motor’s insulation rating wasn't rated for the voltage spikes a modern VFD generates. Within a week, we had a winding failure. Cost us a ton of time and a rush order on a replacement motor (ugh).

Here’s the deal: Most modern VFDs use PWM (Pulse Width Modulation) which can create voltage spikes that degrade old motor insulation. Furthermore, many older motors have a non-inverter-duty fan that doesn't cool effectively at low speeds. So, my rule of thumb now: If the motor is over 10 years old, or if the nameplate doesn't explicitly say 'Inverter Duty' or list a 'VFD compatible' spec, you're better off budgeting for a new motor. It’s way cheaper than the downtime.

Q: So when does it make sense to pair an old motor with a new VFD?

Great question. There are exceptions. If you have a higher-quality industrial motor that's in excellent shape and has a service factor of 1.15 or higher, and you're only planning to use the VFD for a narrow speed range (like 80-100% of full speed), you might be fine. I personally have a mix of old and new on my own projects. For a simple fan application where we just wanted to trim airflow by 10%, the old motor handles it fine.

But then again, when you need to go below 50% speed for a constant-torque load (like a conveyor), the risk goes way up. That's when the old motor's cooling becomes a serious problem. So basically, know your application. If you're not sure, a call to your motion control distributor's application engineer is a super smart investment. Most good ones will provide this support as part of the sale.

Q: I'm being quoted for 'servo motors and drives' vs. a VFD setup. Why would I pay way more for that?

It took me about 150 different projects and a few expensive mistakes to understand this trade-off. The short answer: servo systems are for precision and torque control at low speeds, while VFDs are for general speed control. A servo motor has an encoder that tells the drive exactly where the motor shaft is. That feedback lets the drive hold position and maintain precise torque even when stopped.

If you're building a CNC machine or a robot arm, you need servos. If you're just running a conveyor or a fan, a VFD and standard motor is usually the right choice. The ballpark cost difference is significant: a servo system can be 3-5x more expensive per axis. But, for a precision application, it’s a no-brainer. Trying to use a VFD for precise positioning is like trying to park a car by throwing it into neutral and hoping for the best.

Q: What about Boston Gear speed reducers? I need to replace one, but the specs are old. Any tips?

Yes, and this is where I see a ton of process gaps. People lose the old spec sheet, or the nameplate is worn off. They then try to match it by eyeballing the shaft size and mounting pattern. I've seen this go wrong so many times.

Here’s my checklist: Don't just measure shaft diameter and overall length. You need the exact ratio, the frame size, the input horsepower rating, and the service factor. The ratio is on the nameplate (e.g., '20:1'). The frame size is a series of letters and numbers (like 'A105' or 'C102'). If you can't find it, measure the center distance between input and output shafts, and the overall mounting footprint. Your best bet is to call a distributor that specializes in Boston Gear. They can cross-reference a lot of old data. I keep a PDF of the Boston Gear catalog on my phone for this exact reason (this was back in 2024, at least).

Q: Why are there different types of linear actuators? I just need to push something.

Fair question! The way I see it, it breaks down into three main types for 99% of applications: electromechanical (screw-driven), electric (belt-driven), and pneumatic/hydraulic. For most industrial motion control, you're looking at the first two.

• Screw-driven (ball screw or lead screw): Best for high precision, high force, and holding a position. Less efficient, but way more rigid. Great for pushing a die or positioning a component.
• Belt-driven: Fast, efficient, and cheaper. But less precise and can't handle high thrust in the same way. Great for pick-and-place or fast transfer applications.
• Pneumatic/Hydraulic: Good for very high forces but are 'bang-bang' controls—hard to position precisely. Usually avoided for modern electronic motion control if you can help it.

So, bottom line: start with what force you need, then what speed, then what accuracy. That will narrow it down to one type pretty quickly.